For contactless detection of torque applied to a rotary shaft of a rotary drive system in electric motors, machine tools, automobiles, and the like, magneto-strictive torque sensors are widely used in which an alternating field is applied to the surface of the shaft so that a change in the magnetic permeability of the shaft surface which arises from application of torque to the rotary shaft is detected in terms of quantity of electricity.
One known type of such magneto-strictive sensor is of the magnetic head system in which a coil winding (a magnetic head) having an open magnetic circuit type iron core, such as U-shaped iron core, is disposed adjacent the shaft to serve as exciting and detecting means, whereby a change in the magnetic permeability of the shaft surface in the direction of principal stress (a direction having an angular inclination of .+-.45 deg. relative to the axial direction of the shaft) which arises from application of torque to the shaft is detected ("Magnetic Measurement of Torque in a Rotating Shaft", The Review of Scientific Instruments, Vol. 25, No. 6, June 1954). Another known type is the solenoid coil system in which uniaxial magnetic anistrophy imparted to the surface of a rotary shaft in an axially inclined direction forms an axis of easy magnetization and, in proximity to the shaft portion to which the magnetic anisotropy is imparted, an exciting solenoid coil (exoiting coil) and a solenoid coil for detecting changes in magnetic permeability (deteoting coil) are arranged around the shaft so that an axial change in the magnetic permeability which is caused by torque application is detected ("A New Torque transducer using Stress Sensitive Amorphous Ribbons", IEEE Trans. on Mag., MAG-18, No. 6, 1769-9, 1982).
For imparting magnetic anisotropy to the surface of the rotary shaft in such torque sensor of the solenoid coil system, there is known a method in which, as described in Japanese Patent Laid Open Publication No. 63-252487, for example, a plurality of belt-like hardened sections parallel to one another in a direction having an angular inclination relative to the axial direction of the shaft are formed on the shaft surface by laser quenching to create residual stress so that, as the effect of the stress, magnetic anisotropy is imparted to the shaft surface. Among other known methods there is one described in Japanese Patent No. 169326, for example, in which a plurality of spiral grooves parallel to one another are formed on the shaft surface by rolling or otherwise so that, as the configurational effect of the grooves, shape magnetic anisotropy is given to the shaft surface.
Changes in the spontaneous magnetization of a ferromagnetic material which are caused by external magnetic field and/or stress acting on the rotary shaft include the process of magnetization due to magnetic domain wall displacement and the process of magnetization due to rotational magnetization which follows the first mentioned domain wall displacement. The process of magnetization due to rotational magnetization is mainly reversible. The process of magnetization due to magnetic domain wall displacement includes the process of reversible magnetization and the process of irreversible magnetization. Unless the external magnetic field and/or active stress is extremely small or unless it is noticeably large, the process of magnetization due to magnetic domain wall displacement is irreversible. This is attributable to the fact that magnetic domain wall displacement is hampered by microfine foreign matters (impurities), grain boundaries, lattice defects, etc. which are present in multiplicity in the ferromagnetic material. Unlike the process of reversible magnetization, the process of magnetization in which magnetic domain wall displacement is largely non-reversible involves hysteresis.
In actual torque sensors, magnetic field applied to the surface of the rotary shaft is usually faint (generally not more than 10 Oe) and, therefore, the process of magnetization on the surface of a conventional rotary shaft made of a magnetically soft material of high strength (coercive force Hc: about 10-20 Oe) is largely of irreversible magnetization due to magnetic domain wall displacement. As such, hysteresis cannot be avoided in the process of magnetization. The hysteresis in the process of magnetization appears as hysteresis in the detection characteristics of the torque sensor, thus adversely affecting the accuracy of torque detection.
On the surface of the rotary shaft there are present not a few defects, such as burrs and hair cracks, though extremely small, which were produced in the machining and groove forming stages. These microdefects can be a cause of hysteresis in the torque detection characteristics or reduced sensitivity. Fatigue of the material of the rotary shaft due to repeated application of torque to the shaft, and intergranular slip due to application of large torque are also likely to cause increased hysteresis in torque detection and reduced detection sensitivity.